The present invention relates to a composition of matter and a method of using such composition in drilling operations and more particularly relates to weighted drilling fluids and a method of forming and maintaining such fluids to aid in drilling operations in subterranean formations.
A variety of compositions used to aid in the production and removal of cuttings (particles released during drilling) from a borehole in the earth are commonly referred to as drilling fluids or drilling muds. For example, in the drilling of an oil well, a drilling fluid is typically circulated down the borehole and back to the surface in a continuous loop. The primary functions of the fluid are to cool and lubricate the drill bit, stabilize uncased sections of the borehole, transport cuttings back to the surface and prevent the inflow of pressurized fluids like gas, oil or water from penetrated formations.
One physical property of a drilling fluid critical to carrying out many of these functions is its density. The density and depth a column of drilling fluid determine the pressure exerted by the drilling fluid on the surrounding formation. In practice, the density of a drilling fluid is usually carefully controlled to exert sufficient downhole pressure to stabilize the walls of the borehole and prevent the inflow of oil or gas from formations that are pierced. In the extreme case, failure to maintain adequate fluid density can lead to a catastrophic, uncontrolled inflow of gas or oil known as a xe2x80x9cblow-outxe2x80x9d.
In many drilling situations, it is necessary to increase the density of the fluid by adding so-called xe2x80x9cweight materialsxe2x80x9d. Weight materials are various inert, high-density particulate solid materials with a normal particle size smaller than 75 microns and preferably smaller than 50 microns. Finely ground barite, hematite and ilmenite are some example of known weight materials.
Because large quantities of weight materials are often added to drilling fluids, the impact of a weight material on the flow properties (such as viscosity) of a drilling fluid is of major importance. It is necessary that the required amount of weight material can be added with a minimum adverse increase in the resistance to flow of the fluid. There is also an upper limit to the amount of weight material that can be added to a drilling fluid before it becomes too thick to be of any practical use.
Various approaches have been devised to provide the desired density with a minimum thickening of the drilling fluid. One obvious approach has been to utilize weight materials of increasingly higher specific gravity to lower the quantity of weight material needed to achieve a given drilling fluid density. Another method is disclosed in U.S. Pat. No. 5,007,480 for using a manganomanganic oxide (Mn3O4) particulate material having a particle size of at least 98% below 10 microns in combination with conventional weight materials such as barite to obtain a drilling mud with a higher density than what is possible using barite alone. Another common approach is to add chemical dispersants to thin the drilling fluid.
It is desired to find an improved weight material and a method of providing such weight material that minimizes the adverse impact on flow properties that increasing additions of weight materials may have on a drilling fluid and increases the maximum density achievable before the fluid becomes too thick.
It is, therefore, the object of the present invention to provide an improved weight material, that shows good property characteristics in drilling fluids, by removing by classification some or all of the finest particles from a finely ground barite and then monitoring and maintaining the selected size by adding coarser material as the particle size degrades during use.
As a drilling operation in subterranean formation progresses, a drilling fluid is pumped into the borehole such that is circulates around the drill bit thus cooling the drill bit. The drilling fluid then carries the cuttings from the drilling operation back up through the borehole to the surface where the cuttings are removed from the drilling fluid so that the drilling fluid can be recirculated into the borehole. Additives are added to the drilling fluid for various reasons such as to provide a weight factor to provide a drilling fluid of a desired density to exert sufficient downhole pressure to stabilize the walls of the borehole and prevent the inflow of oil, gas and other materials from surrounding formations.
In the present invention, a material such as barite is finely ground (using conventional grinding methods) to a particle size (xe2x80x9cthe selected sizexe2x80x9d) typical of weight materials presently in commerce and usually meeting the specifications for barite used as a drilling fluid material established by the American Petroleum Institute (API) in API Specification 13A, Section 2. The barite is subsequently classified to remove some or all of the finest particles to improve its performance in drilling fluid.
In the preferred embodiment of the invention, the classified barite contains about 0-5% by weight of particles larger than about 75 microns and about 0-10% by weight of particles less than about 6 microns as determined using American Petroleum Institute test methods. It is worthy of note that, depending on the size and quantity of fine particles removed, the classified barite may no longer meet the API specifications.
The classification can be accomplished by any of several known methods for separating particulate materials into separate coarse and fine fractions. One example of a suitable classification system is taught in U.S. Pat. No. 5,366,095 where a rotating turbine is used to impart centrifugal force on particles that are suspended in an air stream. Larger particles are thrown radially away from the airflow and are captured by the device while smaller particles of insufficient mass to be rejected by the centrifugal force remain entrained in the airstream and are separately recovered. Adjustment of the airflow and speed of rotation of the turbine allows for efficient separations at a desired particle size.
The classified barite material is then added to the drilling fluid to obtain the desired density. During the circulation of the drilling fluids containing the selected-size barite particles, the particles may degrade into a smaller size. Therefore, to maintain the selected size of barite present in the drilling fluid, a coarser grind of barite material can be periodically added to the drilling fluid.